Non-Conservation of Angular Momentum - How does Torque Come into Play?

Okay, here is a rotational motion problem that is baffling me:

We are given an object's (specifically, a wheel's) moment of inertia, an initial rpm, and a final rpm. The question is: What (constant) torque is required to produce the angular acceleration?

No time is mentioned at all, so

(torque) = (moment of inertia)(angular acceleration)

is out. I suspect that I must use angular momentum somehow. However, the book (Schaum's Outline series) provides no specific examples, except in the case of conservation of angular momentum. On the net there are tons of examples of conservation of angular momentum when there is no net torque acting on the object. But I can't find any examples to tie together

(final angular momentum) - (initial angular momentum)

and torque.

This is driving me nuts.